1. Prior Art
The present invention relates to an organic positive temperature coefficient thermistor that is used as a temperature sensor or overcurrent-protecting element, and has PTC (positive temperature coefficient of resistivity) characteristics or performance that its resistance value increases with increasing temperature.
2. Background Art
An organic positive temperature coefficient thermistor having conductive particles dispersed in a crystalline thermoplastic polymer has been well known in the art, as typically disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882. The increase in the resistance value is thought as being due to the expansion of the crystalline polymer upon melting, which in turn cleaves a current-carrying path formed by the conductive fine particles.
An organic positive temperature coefficient thermistor can be used as a self control heater, an overcurrent-protecting element, and a temperature sensor. Requirements for these are that the resistance value is sufficiently low at room temperature in a non-operating state, the rate of change between the room-temperature resistance value and the resistance value in operation is sufficiently large, and the resistance value change upon repetitive operations is reduced.
To meet such requirements, it has been proposed to use a low-molecular organic compound such as wax and employ a thermoplastic polymer matrix for a binder. Such an organic positive temperature coefficient thermistor, for instance, includes a polyisobutylene/paraffin wax/carbon black system (F. Bueche, J. Appl. Phys., 44, 532, 1973), a styrenebutadiene rubber/paraffin wax/carbon black system (F. Bueche, J. Polymer Sci., 11, 1319, 1973), and a low-density polyethylene/paraffin wax/carbon black system (K. Ohe et al., Jpn. J. Appl. Phys., 10, 99, 1971). Self control heaters, current-limiting elements, etc. comprising an organic positive temperature coefficient thermistor using a low-molecular organic compound are also disclosed in JP-B""s 62-16523, 7-109786 and 7-48396, and JP-A""s 62-51184, 62-51185, 62-51186, 62-51187, 1-231284, 3-132001, 9-27383 and 9-69410. In these cases, the resistance value increase is believed to be due to the melting of the low-molecular organic compound.
One of advantages to the use of the low-molecular organic compound is that there is a sharp rise in the resistance increase with increasing temperature because the low-molecular organic compound is generally higher in crystallinity than a polymer. A polymer, because of being easily put into an over-cooled state, shows a hysteresis where the temperature at which there is a resistance decrease with decreasing temperature is usually lower than the temperature at which there is a resistance increase with increasing temperature. With the low-molecular organic compound it is then possible to keep this hysteresis small. By use of low-molecular organic compounds having different melting points, it is possible to easily control the temperature (operating temperature) at which there is a resistance increase. A polymer is susceptible to a melting point change depending on a difference in molecular weight and crystallinity, and its copolymerization with a comonomer, resulting in a variation in the crystallographic state. In this case, no sufficient PTC characteristics are often obtained.
In the organic positive temperature coefficient thermistors set forth in the above publications, however, no sensible tradeoff between low initial (room temperature) resistance and a large rate of resistance change is reached. Jpn. J. Appl. Phys., 10, 99, 1971 shows an example wherein the specific resistance value (xcexa9xc2x7cm) increases by a factor of 108. However, the specific resistance value at room temperature is as high as 104 xcexa9xc2x7cm, and so is impractical for an overcurrent-protecting element or temperature sensor in particular. Other publications show resistance value (xcexa9) or specific resistance (xcexa9cm) increases in the range between 10 times or lower and about 104 times, with the room-temperature resistance being not fully decreased.
A problem associated with using the thermoplastic polymer for the matrix is that because the matrix melts and fluidizes at the melting point of the polymer, the dispersion state of the system changes upon exposure to high temperature in particular, resulting in unstable performance.
On the other hand, JP-A""s 2-156502, 2-230684, 3-132001 and 3-205777 disclose an organic positive temperature coefficient thermistor using a low-molecular organic compound and a thermosetting polymer behaving as a matrix. Since carbon black, and graphite are used as conductive particles, however, the rate of resistance change is as small as one order of magnitude or less and the room-temperature resistance is not sufficiently reduced or about 1 xcexa9xc2x7cm as well. Thus, no compromise is made between the low initial resistance and the large rate of resistance change.
JP-A""s 55-68075, 58-34901, 63-170902, 2-33881, 9-9482 and 10-4002, and U.S. Pat. No. 4,966,729 propose an organic positive temperature coefficient thermistor constructed solely of a thermosetting polymer and conductive particles without recourse to a low-molecular organic compound. In these themistors, either, no compromise is achieved between a room-temperature resistance of up to 0.1 xcexa9xc2x7cm and a large rate of resistance change of 5 orders of magnitude greater, because carbon black, and graphite are used as the conductive particles. Generally, thermistor systems composed merely of a thermosetting polymer and conductive particles have no distinct melting point, and so many of them show a sluggish resistance rise in temperature vs. resistance performance, failing to provide satisfactory performance in overcurrent-protecting element, temperature sensor, and like applications in particular.
In many cases, carbon black, and graphite have been used as conductive particles in prior art organic positive temperature coefficient thermistors including those set forth in the above publications. A problem with carbon black is, however, that when an increased amount of carbon black is used to lower the initial resistance value, no sufficient rate of resistance change is obtainable; no reasonable tradeoff between low initial resistance and a large rate of resistance change is obtainable. Sometimes, particles of generally available metals are used as conductive particles. In this case, too, it is difficult to arrive at a sensible tradeoff between the low initial resistance and the large rate of resistance change.
One approach to solving this problem is disclosed in JP-A 5-47503 that teaches the use of conductive particles having spiky protuberances. More specifically, it is disclosed that polyvinylidene fluoride is used as a crystalline polymer and spiky nickel powders are used as conductive particles having spiky protuberances. U.S. Pat. No. 5,378,407, too, discloses a thermistor comprising filamentary nickel having spiky protuberances, and a polyolefin, olefinic copolymer or fluoropolymer. However, these thermistors are still insufficient in terms of hysteresis and so are unsuitable for applications such as temperature sensors, although the effect on the tradeoff between low initial resistance and a large resistance change is improved. This is because no low-molecular organic compound is used as a working or active substance. Another problem with these thermistors is that when they are further heated after the resistance increase upon operation, they show NTC (negative temperature coefficient of resistivity) behavior that the resistance value decreases with increasing temperature. It is to be noted that the above publications give no suggestion about the use of a low-molecular organic compound at all.
JP-A 5-198403 and 5-198404 disclose an organic positive temperature coefficient thermistor comprising a mixture of a thermosetting resin and conductive particles having spiky protuberances, and show that the rate of change resistance obtained is 9 orders of magnitude greater. However, when the room-temperature resistance value is lowered by increasing the amount of a filler, no sufficient rate of resistance change is obtained. Thus, it is difficult to achieve a tradeoff between low initial resistance value and a large resistance change. Also, the thermistors fail to show a sufficiently sharp resistance rise because of being composed of the thermosetting resin and conductive particles. The above publications, too, are silent about the use of a low-molecular compound.
An object of the invention is to provide an organic positive temperature coefficient thermistor that has sufficiently low resistance at room temperature and a large rate of resistance change between an operating state and a non-operating state, and can operate with a reduced temperature vs. resistance curve hysteresis, no NTC behavior after a resistance increase, ease of control of operating temperature, and high performance stability.
Such an object is achieved by the inventions defined below.
(1) An organic positive temperature coefficient thermistor comprising a thermosetting polymer matrix, a low-molecular organic compound and conductive particles, each having spiky protuberances. (2) The organic positive temperature coefficient thermistor according to (1), wherein said low-molecular organic compound has a melting point of 40 to 200xc2x0 C. (3) The organic positive temperature coefficient thermistor according to (1), wherein said low-molecular organic compound has a molecular weight of 4,000 or lower. (4) The organic positive temperature coefficient thermistor according to (1), wherein said low-molecular organic compound is a petroleum wax or a fatty acid. (5) The organic positive temperature coefficient thermistor according to (1), wherein said thermosetting polymer matrix is any one of an epoxy resin, an unsaturated polyester resin, a polyimide, a polyurethane, a phenol resin, and a silicone resin. (6) The organic positive temperature coefficient thermistor according to (1), wherein a weight of said low-molecular organic compound is 0.2 to 2.5 times as large as a weight of said thermosetting polymer matrix. (7) The organic positive temperature coefficient thermistor according to (1), wherein said conductive particles, each having spiky protuberances, are interconnected in a chain form.
In the present invention, the spiky shape of protuberances on the conductive particles enables a tunnel current to pass readily through the thermistor, and makes it possible to obtain initial resistance lower than would be possible with spherical conductive particles. When the thermistor is in operation, a large resistance change is obtainable because spaces between the spiky conductive particles are larger than those between spherical conductive particles.
In the present invention, the low-molecular organic compound is incorporated in the thermistor so that the PTC (positive temperature coefficient of resistivity) performance that the resistance value increases with increasing temperature is achieved by the melting of the low-molecular organic compound. Accordingly, the temperature vs. resistance curve hysteresis can be more reduced than that obtained by the melting of a crystalline thermoplastic polymer. Control of operating temperature by use of low-molecular organic compounds having varying melting points, etc. is easier than control of operating temperature making use of a change in the melting point of a polymer. Unlike a thermistor using a thermosetting polymer as a working or active substance, the thermistor of the invention shows a sharp resistance rise upon operation.
Further, the present invention uses the thermosetting polymer as the matrix. When the thermistor of the invention is put in operation, the large resistance change is obtained making use of a large volume expansion of the low-molecular organic compound incidental to its melting. However, a thermistor element composed only of a low-molecular organic compound and conductive particles cannot retain shape upon operation because the melting viscosity of the low-molecular organic compound is low. To prevent fluidization of the low-molecular organic compound due to its melting when the thermistor element is in operation or prevent deformation of the thermistor element upon operation, it is thus required to disperse the low-molecular organic compound and conductive particles in the matrix polymer. When a thermoplastic polymer is used for this matrix polymer, a problem arises in conjunction with high-temperature stability in particular because the polymer melts at greater than its melting point. According to the invention wherein the thermosetting polymer is used for the polymer matrix to disperse the low-molecular organic compound and conductive particles in the insoluble and infusible three-dimensional matrix, the thermistor is much more improved in performance stability than a thermistor using a thermoplastic polymer, and so the thermistor can maintain the low room-temperature resistance and the large resistance change upon operation over an extended period of time.
When a thermistor using a thermoplastic polymer matrix is heated after its resistance has increased, there is found an NTC phenomenon in which the resistance value decreases with increasing temperature. Upon cooling, the thermistor shows a large temperature vs. resistance curve hysteresis that is the resistance decreases from a temperature higher than the melting point of the low-molecular organic compound. The fact that a thermistor is restored in resistance value at a temperature higher than the preset temperature can become a serious problem when it is used especially as a protective element. The NTC phenomenon is also found in a system using a thermoplastic resin and conductive particles. The resistance decrease appears to be because of the realignment of the conductive particles in the matrix in a molten state by a current continuing to pass through the thermistor even after a resistance increase. The same reason may also hold for the case where, upon cooling, the resistance value decreases from a temperature higher than the operating temperature upon heating. According to the present invention, the above problems, i.e., the NTC phenomenon occurring after the resistance increase and the temperature vs. resistance curve hysteresis, can be substantially eliminated by use of the insoluble and infusible thermosetting polymer matrix.